Nanowire arrays for trace vapor preconcentration

ABSTRACT

Disclosed herein is a method of providing a structure having two electrodes connected by nanowires, exposing the structure to an analyte that can adsorb onto the nanowires, and passing an electrical current through the nanowires to heat the nanowires to desorb the analyte. Also disclosed herein is an apparatus having the above structure; a current source electrically connected to the electrodes, and a detector to detect the analyte.

This application is a continuation-in-part application of U.S. Pat. No.10,167,192, issued on Jan. 1, 2019, which is a continuation applicationof U.S. Pat. No. 9,422,158, issued on Aug. 23, 2016, claims the benefitof U.S. Provisional Application No. 61/413,664, filed on Nov. 15, 2010.This application claims the benefit of U.S. Provisional Application No.62/486,568, filed on Apr. 18, 2017. These applications and all otherpublications and patent documents referred to throughout thisnonprovisional application are incorporated herein by reference.

TECHNICAL FIELD

The present disclosure is generally related to electrodes that may beused in sensors.

DESCRIPTION OF RELATED ART

Many types of nanowires, and other nanometer-scale structures of similardimensions, have been at the heart of a large research effort aimed atstudying their unique properties and integrating them into noveldevices. For example, many different types of sensors have beenfabricated from either single (Cui et al., Science 293 (2001) 1289) oran array of silicon nanowires (Engel et al., Agnew. Chem. Int. Ed. 49(2010) 6830) to take advantage of the favorable physical, chemical,electrical, and optical properties of nanowires. For many deviceapplications, such as gas sensors, a vertical nanowire orientation isideal since it maximizes the surface area of nanowires that come incontact with the environment (Offermans et al., Nano Lett. 10 (2010)2412), while also minimizing the deleterious effects of substrate oxidesand other surface chemistry. These deleterious effects includetrapping/detrapping of charge carriers, nonselective adsorption of othermolecules on the substrate, and steric denial of part of the nanowire'ssurface to reaction with the target molecule. Furthermore, a largenumber of such nanowires in an array improves device performance byreducing 1/f noise and other noise types sensitive to the number ofcarriers.

A challenge in creating a sensor type device based on vertical nanowirearrays lies in making individual electrical connections to all thenanowires. The few existing approaches have involved embedding theentire nanowire array in some type of a sacrificial material, exposingthe tips of the nanowires, and depositing the desired top contactelectrode layer (Offermans et al., Nano Lett. 10 (2010) 2412; Park etal., Nanotechnology 19 (2008) 105503; Peng et al., Appl. Phys. Lett. 95(2009) 243112). In these cases, the nanowire sensing region is exposedupon removal of the sacrificial material, and the substrate itselfserves as the bottom electrode. Methods based on the deposition of aporous gold nanoparticle film on top of the nanowire array (Parthangalet al., Nanotechnology 17 (2006) 3786) and the random gap-bridging ofnanowires during growth (Ahn, et al., Appl. Phys. Lett. 93 (2008)263103) have also been investigated. In all these approaches, anon-ordered array of vertical nanowires was used as the main sensingelement. More importantly, none of these methods are able to create aporous top contact electrode layer with holes of controllable size anddistribution.

While various methods exist for creating porous electrodes (Lohmuller etal., J. Micromech. Microeng. 18 (2008) 115011; Kim et al., Sens.Actuators, B 141 (2009) 441-446), these methods are primarily designedfor simply increasing the surface area of the electrode and are notapplicable for creating such structures on top of a nanowire array.Other attempts, such as gold nanoparticle films (Parthangal et al.,Nanotechnology 17 (2006) 3786) and electrospun metal nanofibers (Wu etal., Nano Lett. 10 (2010) 4242), do not allow precise control over thesize and placement of holes in the electrode layer while also beingsignificantly limited in the types of materials that can be used.

Accurate, reliable standoff sensing of trace vapors in complexenvironments, such as explosives associated with improvised explosivesdevices (IEDs), is both a critical need and a significant challenge. Theproblem is akin to that of locating the proverbial needle in a haystack:trace explosives vapors, present at concentrations several orders ofmagnitude below their saturated vapor concentration, must be sensed inenvironments containing other non-target vapors present at many ordersof magnitude higher concentration.

A number of technologies have been brought to bear on this problem andcan be loosely categorized as 1) spectroscopy-based, 2) sensor-based, or3) traditional analytical approach-based. Each technology has associatedpros and cons, as discussed briefly below.

Optical measurements of trace explosives vapors are intrinsicallydifficult in the gas phase due to the low vapor pressures associatedwith analytes of interest (μTorr and below, with realized vaporconcentrations below ppb-levels) and the high limits of detection (LODs)attributed to these techniques. Numerous spectroscopic techniques havedemonstrated capability for detecting explosives vapors, however, inmany cases at concentrations that are not representative of a real worldscenario (Johansson et al., “Stand-off detection of explosives vapors byResonance enhanced Raman spectroscopy” Proceedings of SPIE, 8709 (2013)87090N, 1-10; Foltynowicz et al., “Terahertz adsorption measurements forgas-phase 2,4-dinitrotoluene from 0.05 THz to 2.7 THz” Chemical PhysicsLetters, 431 (2006), 34-38; Zhang et al., “Detection of gas-phaseexplosive analytes using fluorescent spectroscopy of thin films ofxanthene dyes” Sensors and Actuators B: Chemical, 225 (2016), 553-562;Todd et al., “Application of mid-infrared cavity-ringdown spectroscopyto trace explosives vapor detection using a broadly tunable (6-8 μm)optical parametric oscillator” Applied Physics B: Lasers and Optics, 75(2002), 367-376). For example, resonance enhanced Raman spectroscopy hasbeen used to detect vapors associated with nitromethane (NM) andmononitrotoluene (4-NT) at distances of 11-13 meters (Johansson). Bothcompounds have vapor pressures that are orders of magnitude abovetraditional military explosives, such as RDX and TNT, yet it was stillnecessary to heat the sample (NM was heated to ˜55° C., and 4-NT washeated to ˜100° C.) in order to enrich the surrounding air withsufficient vapor for detection. In fact, the authors note that whilesuch an approach is functional in laboratory settings, it is difficultto implement in real scenarios due to the low concentration of vaporsurrounding bulk explosives. Terahertz spectroscopy-based techniqueshave also been demonstrated on explosives vapors, including efforts byFoltynowicz and coworkers (Foltynowicz). They demonstrated the firstgas-phase spectrum for 2,4-dinitrotoluene using pulsed terahertztime-domain spectroscopy. As with resonance enhanced Raman spectroscopy,it was necessary to heat the sample, in this case to as much as 150° C.,to increase the saturated vapor pressure from 147 μTorr at roomtemperature to 19 Torr.

The primary spectroscopic successes are for the detection of traceparticulate solids on surfaces (Bremer et al., “Standoff explosivestrace detection and imaging by selective stimulated Raman scattering”Applied Physics Letters, 103 (2013), 061119, 1-5; Kendziora et al.,“Infrared photothermal imaging for standoff detection applications”Proceedings of SPIE, 8373 (2012) 8373H, 1-10; Almaviva et al., “A neweye-safe UV Raman spectrometer for the remote detection of energeticmaterials in fingerprint concentrations: Characterization by PCA and ROCanalysis” 144 (2015), 420-426; Jha et al., “Towards deep-UVsurface-enhanced resonance Raman Spectroscopy f explosives:ultrasensitive, real-time reproducible detection of TNT” Analyst, 140(2015), 5671-5677; Galan-Freyle et al., “Standoff detection of highlyenergetic materials using laser-induced thermal excitation of infraredemission” Applied Spectroscopy, 69 (2015), 535-544). Bremer and Dantusdemonstrated laser-based standoff detection of nanograms of NH₄NO₃ andTNT explosives using stimulated Raman scattering (Bremer). Imaging ofsurfaces using this technique enabled standoff detection of particles onboth textured plastic and cotton at distances up to 10 meters. Infraredphotothermal imaging, in which an IR quantum cascade laser isspecifically tuned to absorption bands in explosives (Kendziora), isanother optical technique for detecting trace solids on a surface. Thesurface is imaged by an IR focal plane array, with detection occurringas a result of increases in the explosives materials temperature due toabsorption of the laser light. Two obvious disadvantages of laser-baseddetection of trace contaminant particles are 1) the requirement that thediameters of the explosives particles on the surface are at least ˜25-50μm, suggesting a “sloppy” bomb maker, and 2) the long time needed tosignal average and scan a large surface area with a laser to detecttrace particle contaminants.

Chemiresistors and microcantilevers are possible chemical sensor typesfor trace explosives. Typical sensor development research focuses onimproving either sensitivity or selectivity (Zhang et al.,“Oligomer-Coated Carbon Nanotube Chemiresistive Sensors for SelectiveDetection of Nitroaromatic Explosives” ACS Applied Materials &Interfaces, 7 (2015) 7471-7475; Wang et al., “Chemiresistive response ofsilicon nanowires to trace vapor of nitro explosives” Nanoscale, 4(2012), 2628-2632; Ray et al., “Development of graphene nanoplateletembedded polymer microcantilever for vapour phase explosive detectionapplications” Journal of Applied Physics, 116 (2014), 124902, 1-5; Leeet al., “Direct detection and speciation of trace explosives using ananoporous multifunction microcantilever” Analytical Chemistry, 86(2014), 5077-5082). For example, Zhang and coworkers recently developeda chemiresistor based on single-wall carbon nanotubes coated with anoligomer (Zhang, ACS Applied Materials & Interfaces). While the sensorsresponded to a suite of common vapors, including acetone, ethanol,hexane, methanol, and toluene, the response was significantly greaterfor nitroaromatics. Their efforts demonstrated TNT detection limits ofapproximately 300 ppt with the addition of the oligomer, enabling themto distinguish TNT from DNT and NT when they included the response fromthe uncoated sensor. Despite efforts to enhance selectivity, however,chemical sensors cannot come close to matching the selectivity oftraditional analytical instrumentation, e.g., mass spectrometry.

Systems-based approaches, such as commercial portable ion mobilityspectrometers (IMS) are suitable for detecting trace explosives solidparticulates collected on swabs, where the analyte is thermally desorbedand delivered to the IMS as a high concentration bolus of analyte. Whilesome vapors are directly detectable via IMS, analytes such as TNT andRDX require preconcentration prior to delivery to the IMS. Martin andcoworkers developed a microfabricated vapor preconcentrator coupled to aportable ion mobility spectrometer, realizing an order of magnitudeimprovement in sensitivity after 15 minutes of sampling (Martin et al.,“Microfabricated vapor preconcentrator for portable ion mobilityspectroscopy” Sensors and Actuators B, 126 (2007) 447-454. The singlegreatest challenge facing chemical sensor research is the difficulty ofintroducing selectivity to these microdevices sufficient to preventsignificant problems associated with false negatives or false positive.

Ultimately, issues associated with selectivity and sensitivity forspectroscopic and sensor-based sensing methods mandate a focus on moretraditional approaches, with an eye towards understanding the mechanismsthat make these instruments so successful.

Traditional chemical detection using analytical instrumentation (i.e.gas chromatography-mass spectrometry (GC-MS)) offers robustcapabilities, but at the cost of instrument size and hardwarecomplexity. For example, current state of the art instrumentsincorporate vapor sampling onto a sorbent material followed by thermaldesorption to a programmable temperature vaporization (PTV) inlet tofocus the desorbed vapor for subsequent introduction to a GC forseparation (Field et al., “Characterization of thermal desorptioninstrumentation with a direct liquid deposition calibration method fortrace 2,4,6-trinitrotoluene quantitation” Journal of Chromatography A,1227 (2012) 10-18; Field et al., “Direct liquid deposition calibrationmethod for trace cyclotrimethylenetrinitramine using thermal desorptioninstrumentation” Journal of Chromatography A, 1282 (2013) 178-182). Thismethodology achieves detection at environmentally relevant levels,however, analysis time can be long (minutes to hours) due to acombination of sampling time (limited by sample flow rate through thestationary phase media) and instrument duty cycle (limited by the timenecessary to desorb and refocus vapor on the PTV inlet). Onemodification of this approach to vapor detection is the direct deliveryof materials desorbed from a reduced stationary phase volume to the headof a GC column, thus removing the PTV inlet (Giordano et al., “DynamicHeadspace-Based Generation and Quantitation of Triacetone TriperoxideVapor” J. Chrom. A, 1331 (2014) 38-43. Two advantageous consequences ofthis modification are increased sampling flow rates and more rapiddesorption due to the reduction in stationary phase volume.

The reduction in total analysis time afforded by increased sampling rateand direct desorption onto a GC column sacrifices selectivity andsensitivity, specifically measurement precision and vapor concentrationat the detector. The primary reason for the degradation is thecontribution of eddy diffusion to broadening of the desorbed analytevapor cut. Eddy or multipath diffusion results from random movements asthe analyte migrates through the packed sorbent bed. The resultingbroadening is exacerbated by radial temperature gradients during heatingof the sorbent bed. Eddy diffusion can reduce the sample concentrationfor a desorbed zone of analyte by several orders of magnitude, causingsignificant errors in elution/migration time.

BRIEF SUMMARY

Disclosed herein is a method comprising: providing a structurecomprising: a first electrode; a plurality of nanowires perpendicular tothe first electrode, each nanowire having a first end in contact withthe first electrode; and a second electrode in contact with a second endof each nanowire; exposing the structure to a sample suspected ofcontaining an analyte that can adsorb onto the nanowires; and passing anelectrical current through the nanowires to heat the nanowires to atemperature at which the analyte will desorb from the nanowires.

Also disclosed herein is an apparatus comprising: a structurecomprising: a first electrode; a plurality of nanowires perpendicular tothe first electrode, each nanowire having a first end in contact withthe first electrode; and a second electrode in contact with a second endof each nanowire; a current source electrically connected to the firstelectrode and the second electrode; and a detector configured to detectan analyte that may be desorbed from the nanowires.

BRIEF DESCRIPTION OF THE DRAWINGS

A more complete appreciation of the invention will be readily obtainedby reference to the following Description of the Example Embodiments andthe accompanying drawings.

FIGS. 1A-L show schematic illustrations of cross-sectional andperspective views of the structure at various stages in the fabricationprocess: FIG. 1A—bar support; FIG. 1B—close-packed monolayer ofnanospheres; FIG. 1C—nanospheres with reduced diameters; FIG. 1D Aucoating on the entire structure; FIG. 1E Au etch template for Sietching; FIG. 1F vertical SiNW array; FIG. 1G vertical SiNW array withthe Au removed; FIG. 1H exposed tips of the SiNW array embedded inphotoresist; FIG. 1I second layer of nanospheres occupying gaps in theSiNW array; FIG. 1J second layer of nanospheres withoxygen-plasma-reduced diameters; FIG. 1K Au coating on the entirestructure; and FIG. 1L completed device showing the PTE and the SiNWarray underneath.

FIGS. 2A-G show scanning electron microscope (SEM) images of thestructure at various stages in the fabrication process: FIG.2A—close-packed monolayer of polystyrene nanospheres; FIG.2B—nanospheres with oxygen-plasma-reduced diameters; FIG. 2C—Au etchtemplate for Si etching; FIG. 2D—vertical SiNW array; FIG. 2E—exposedtips of the SiNW array embedded in photoresist; FIG. 2F—second layer ofnanospheres perfectly occupying gaps in the SiNW array; and FIG.2G—completed device showing the PTE and the SiNW array underneath.

FIG. 3 shows a schematic diagram of completed device (top view).

FIGS. 4A-B show sensor response to various concentrations of NO₂ and NH₃following 2 min of clean air: FIG. 4A—1 ppm of NH₃, 500 ppb of NH₃, 1ppm of NO₂, and 500 ppb of NO₂ at ˜30% RH; and FIG. 4B—250 ppb of NO₂,50 ppb of NO₂ and 10 ppb of NO₂ at <10% RH.

FIG. 5A shows PTE sensor and solid electrode sensor response to 500 ppbof NH₃ at ˜30% RH, and FIG. 5B shows the delayed saturation response ofthe solid electrode sensor.

FIG. 6 shows sensor response to ammonia and nitrogen dioxide at variousconcentrations. The dashed line is an extension of the baseline forcomparison.

FIG. 7 shows the calibration curves for (A) ammonia and (B) nitrogendioxide using an initial slope-based method and the calibration curvesfor (C) ammonia and (D) nitrogen dioxide using a fixed-time point methodwith |ΔR/R₀|_(saturation).

FIG. 8 shows an SEM side view of SiNW array.

FIG. 9 shows an optical side view of array. The bright spot indicatesthe Raman laser.

FIG. 10 shows the Raman spectra as a function of applied current.

FIG. 11 shows the temperature versus current relationship.

FIG. 12 shows current titration desorption beginning at 10 mAmps to 200mAmps, at 10 mAmp increments (200 mAmp is repeated once). Current wasapplied for 8 seconds, with 30 seconds between each application. A 28.4ppb_(v) vapor was sampled for 5 minutes at 200 mL/min providing anominal sample load of 211 ng of 2,4-DNT. Mass selective detection atm/z=77, 89, 123, 165.

FIG. 13 shows a separation of nitrobenzene (NB) and 2,4-DNT.

FIG. 14 shows a separation of nitrobenzene, 2,6-DNT, and 2,4-DNT.

FIG. 15 shows a separation of 2,6-DNT and TNT.

DETAILED DESCRIPTION OF EXAMPLE EMBODIMENTS

In the following description, for purposes of explanation and notlimitation, specific details are set forth in order to provide athorough understanding of the present disclosure. However, it will beapparent to one skilled in the art that the present subject matter maybe practiced in other embodiments that depart from these specificdetails. In other instances, detailed descriptions of well-known methodsand devices are omitted so as to not obscure the present disclosure withunnecessary detail.

Microfabricated sensors based on nanostructures such as spheres, wires,rods, tubes, and ribbons have been the focus of intense research in aneffort to achieve field deployable, gas or liquid phase sensors fordetection of chemical warfare agents and explosives. Such sensors wouldbe selective and sensitive, miniature, low power, fast, economical,simple-to-use, and capable of detecting a wide range of analytes incomplex environments such as a battlefield or an airport. The uniqueelectrical and mechanical properties of nanostructures give them greatpotential but also problems in gas phase sensing platforms such aschemical field-effect transistors (ChemFETs). For example, prototypenanoscale devices are more sensitive to analyte adsorption thanmacroscale bulk devices because of their high surface-to-volume ratios.However, they also have relatively poor signal-to-noise ratios due toshot noise and 1/f noise, which are more significant at the nanoscale.Single nanowires can respond quickly to the analyte; however,diffusion-limited mass transport through a nanowire array preventssimultaneous response by all of the nanowires and hence increasesresponse time. A good nanostructure-based gas sensor maximizes thesurface area of the sensing element, reduces or eliminates chargecarrier related noise sources, and minimizes diffusion-hindered responsetime.

Silicon nanowires may meet the requirements of such an idealnanostructure-based sensor. They are easy to fabricate with existingsilicon fabrication techniques that reduce cost and ensure integrabilitywith conventional CMOS devices. Vertical arrays offer significantadvantages by minimizing major noise sources at the nanoscale andmaximizing sensor surface area; noisy wire-to-wire junctions areeliminated and the wire surface is not blocked by the supportingsubstrate. Additionally, vapor diffusion through vertically alignedsilicon nanowire arrays is critical because hindered diffusion increasesthe response time.

Disclosed herein is a method for creating arrays of vertical nanowires,especially ordered arrays, either with a solid top electrode or a topelectrode with an array of holes, especially a periodic and well-definedarray of holes. The holes in the top contact electrode layer may allowvarious elements, such as gases or liquids, to flow rapidly through itand come in contact with the sensing nanowire region underneath. Theholes or perforations may be sized and located such that electricalcontact will be established to the tips of the nanowires in the arraywhile maximizing the overall porosity of the electrode layer. In thecase of the ordered arrays, the periodic placement may maximize theinflux of gas or liquid from the side of the wires comprising the array.In some configurations, there may be clear channels all the way throughthe array. Likewise, the nanowires in an ordered array usually havesimilar or identical dimensions and pitch, thus minimizing wire to wirevariations and allowing selection of the dimensions giving the bestresponse. Disordered nanowire arrays may still benefit from the poroustop electrode, which provides another avenue for rapid target moleculeingress to all of the nanowires comprising the array.

The support and nanowires can be any material that is compatible withthe electrical measurement to be performed, including but not limited tosemiconducting, conducting, metallic, or insulating material. There maybe an electrical connection between the nanowires and the support. Oneexample support material is silicon, such as a silicon wafer. Thesupport may be a substrate or another electrode, including theperforated electrode described herein. The support may include anelectrical contact on the surface opposed to the nanowires.

The nanowires may be made of the same material as the support or of adifferent material, and may be, for example, silicon, single-wall carbonnanotubes, multi-wall carbon nanotubes, or gallium nitride. Any materialthat can be made in the vertical nanowire array configuration may beused. Another option is an array of core-shell nanowires. For example,the array consists of Si nanowires that are coated with another materialthat is more susceptible to Joule heating than Si, so that the shellgets hot, while the Si nanowires themselves function only as a platform.The properties of the nanowire material may be either controlled or not.In the case of controlled material, this includes, for example,composition, doping and electrical conductivity, crystallinity, chemicalfunctionalization, and additional surface layers.

There are a plurality of nanowires that are perpendicular to the supporthaving only the second end in contact with the support. However,additional nanowires that are not perpendicular to the support may alsobe present. As used herein, “perpendicular” may be defined as within 1°,5°, 10°, 20°, 40°, or 60° of normal to the support. The nanowiredimensions may be either uncontrolled or controlled as to, for example,length, diameter, and crystal face. They may be of uniform length inthat they are all of a length that is within 1%, 5%, 10%, or 20% oftheir average length.

Methods of forming nanowires on a support are known in the art,including but not limited to methods disclosed in Huang et al., Adv.Mater. 23 (2011) 285-308; Kayes et al., Appl. Phys. Lett., 91 (2007)103110; Lee et al., Nano Lett. 10 (2010) 1016-1021; Weisse et al., NanoLett. 11 (2011) 1300-1305; and Offermans et al., Nano Lett. 10 (2010)2412-2415. The nanowires and support may both be made from the sameprecursor substrate. This may be done by etching the precursor substrateto leave behind the nanowires and the support. Other methods include,but are not limited to, growing the nanowires on the support andattaching pre-formed nanowires to the support. Growth methods include,but are not limited to, chemical vapor deposition (catalyzed oruncatalyzed), physical vapor deposition, molecular beam epitaxy andrelated growth methods, and growth in a liquid.

In some embodiments, an ordered array of vertical nanowires can beetched into (Peng et al., Adv. Mater. 14 (2002) 1164) or grown out of(Westwater et al., J. Vac. Sci. Technol. B 15 (1997) 554) a substrate ofvarious materials. The spacing between nanowires as well as theirdiameters can be controlled through a range of methods including, butnot limited to, photolithography, electron beam lithography,interference lithography, and nanosphere lithography. A combination ofnanosphere lithography and catalytic etching of silicon (Peng et al.,Appl. Phys. Lett. 90 (2007) 163123) can quickly yield periodic verticalsilicon nanowire arrays with well-controlled dimensions and materialproperties where every nanowire has approximately the same diameter.

The nanowires may be randomly arranged or periodically arranged on thesupport, such as, for example, a hexagonal arrangement of nanowires. Onemethod to form periodic nanowires is to deposit a close-packed hexagonalarray of nanospheres on a precursor substrate, etch the nanospheres tomake them smaller and expose portions of the substrate between thenanospheres, deposit an etching catalyst on the nanospheres and exposedprecursor substrate, removing the nanospheres, and etching thesubstrate. This produces a hexagonal array of nanowires of approximatelyequal length, of the same pitch as the close-packed array ofnanospheres, and of a diameter approximately the same as thereduced-size nanospheres. Other nanoparticles may also be used to formother arrangements of nanowires. For example, nanoparticles ornanospheres ranging in size from 50 nm to 1 μm in diameter can be used,as well as larger and smaller sizes. The electrode may be made of anymaterial that is compatible with the electrical measurement to beperformed. It may be any metal or other conducting material such as atransparent conducting oxide or a film of nanotubes or othernanostructures. The electrode is of any thickness and the holes may beof any diameter and spacing. There may be an electrical connectionbetween the nanowires and the electrode. Example electrodes may bedeposited from a vapor or other method and may form a continuousmaterial. A continuous material is formed as a single article, includinga layered article, rather than as a conglomeration of smaller objectssuch as nanoparticles or entangled filaments. Example electrodematerials include, but are not limited to, a combination of titanium andgold, silver, aluminum, graphene, and a combination of chrome and gold.

The electrode contains perforations, which are open spaces forming astraight line path normal to the support and completely through theelectrode. The perforations may have a diameter that is larger than thethickness of the electrode. The perforations may be randomly arranged orperiodically arranged. The nanowires described above would not haveexposed tips immediately under the perforations, but additional suchnanowires may be present.

One example method for forming the perforations is to deposit a fillermaterial to cover the nanowires with a filler material leaving the firstends of the nanowires exposed. This may be done at the outset or excessfiller material may be removed after completely covering the nanowires.The filler material can be any material that can later be removedwithout removing the nanowires and electrode, including but not limitedto a photoresist, an oxide, alumina, or silica. Nanoparticles are thendeposited on the filler material in the locations to become theperforations. The nanoparticles may be nanospheres in a closed-packedhexagonal array with the tips of the nanowires in the spaces betweennanospheres. Optionally, the size of the nanoparticles may be reduced toallow for smaller perforations. The electrode material is then depositedon top of the entire structure including the nanoparticles, the tips ofthe nanowires, and any exposed filler material. The nanoparticles andfiller material along with the attached unwanted electrode material arethen removed, leaving behind the substrate, nanowires, and perforatedelectrode.

Periodic perforations are formed when using close-packed nanospheres,which may be from a solution containing polystyrene (or similar)nanospheres spun on the sample. Spin-on parameters can be controlled toyield a close-packed monolayer of nanospheres on top of the nanowires.If the nanosphere diameter is equal to the nanowire-to-nanowiredistance, each nanosphere will be geometrically constrained to fill inthe gaps between the nanowires. The nanospheres will be prevented fromresting on the tips of nanowires, which provides automatic alignment ofadditional nanospheres for particles to fill the void between wires. Ifthe nanospheres are large enough, it will not be possible for more thanone nanosphere to occupy the void between nanowires. However, smallernanospheres or nanoparticles may be used to form multiple smallerperforations between adjacent nanowires.

The method may also be used to produce nonperiodic perforations if thenanowires are not periodic, if the nanoparticles are not closely packed,or other types of nanoparticles are used. By omitting the deposition ofnanoparticles on top of the nanowires, non-perforated electrodes can bemade on top of either ordered or non-ordered arrays of nanowires.

The structure may be used as a part of a sensor using a transductionmechanism for converting adsorbed molecules into an electrical signal.An electrical signal can be a change in voltage, current, resistance,frequency, or capacitance. A sensor typically sources (provides) avoltage or current and in turn measures the current or voltage,respectively. The measured value along with the output is used toconvert to a resistance. The structure may be exposed to a sample, andthen a change in an electrical property of the structure is measured.For example, the resistance between the support (or its includedelectrical contact) and the electrode may change in response to one ormore analytes. Examples of the application of such sensors include thedetection of gas or liquid-borne explosives and chemical or biologicalagents or toxic industrial chemicals (TICs).

The following steps may be performed to form a structure.

Nanowire Formation

-   1. Start with a p-type silicon wafer with resistivity of    approximately 1˜10 Ω-cm.-   2. Perform the following cleaning steps at room temperature (FIG.    1A):    -   30 minutes in 3:1 solution of H₂SO₄ and 30% H₂O₂    -   30 minutes in 5:1:1 solution of H₂O, NH₄OH, and H₂O₂-   3. Deposit 490 nm polystyrene nanosphere solution (10% solids) on    sample and spincoat to achieve close-packed monolayer (approximately    1 μL of nanosphere solution per 1 cm² of substrate) (FIG. 1B).-   4. Allow sample to dry overnight.-   5. Reduce nanosphere diameter to desired value using an oxygen    plasma etch (FIG. 1C).-   6. Deposit 25 nm of gold on top of the sample using an e-beam    evaporator (FIG. 1D).-   7. Remove nanospheres and unwanted metal by soaking ˜5 minutes in    CHCl₃ (FIG. 1E). Brief sonication may be necessary.-   8. Etch the sample in a solution of 4.6 M HF and 0.44 M H₂O₂ for    20˜30 minutes for nanowires around 4˜8 μm in length (FIG. 1F).-   9. Remove the remaining gold using a TFA gold etchant (FIG. 1G).-   10. Carefully rinse and dry the sample using a critical point dryer.    Electrode Formation-   11. Deposit a thick layer of photoresist to entirely cover the    nanowire array.-   12. Remove the top layer of the photoresist layer using an oxygen    plasma etch to reveal the nanowire tips (FIG. 1H).-   13. Deposit 490 nm polystyrene nanospheres using the same process    shown in step 3 (FIG. 1I).-   14. Reduce nanosphere diameter to desired value using an oxygen    plasma etch (FIG. 1J).-   15. Deposit the electrode layer consisting of 20 nm of titanium and    100 nm of gold using an e-beam evaporator (FIG. 1K).-   16. Soak the sample overnight in acetone to remove the photoresist    and nanosphere layers (FIG. 1L). Brief sonication and/or soak in    CHCl₃ may be necessary to completely remove the nanospheres.-   17. Dry the sample using a critical point dryer.

In another embodiment, the nanowires are immobilized in a fillermaterial, and then removed from the support as a unit, exposing thesecond ends of the nanowires. The filler material may be any materialthat holds the nanowires in place and can later be removed, such as apolymer or the filler materials described above. Any of the supports,substrates, nanowires, nanoparticles, electrodes, and processesdescribed herein may be used in this embodiment.

The support may be removed from the nanowires and filler material beforeor after a perforated electrode is formed on the first, exposed side ofthe structure. In one variation, the support is removed and a perforatedelectrode is formed on the first side, followed by forming a secondelectrode on the second side. The second electrode may cover the entiresecond side or may be perforated by the same method as the firstelectrode. The second electrode may also be formed before the first.Alternatively, the first electrode is formed, then the support isremoved, then the second electrode is formed. When the electrodes areformed separately, the filler material may remain present for theformation of both, or it may be removed after forming one electrode andreplaced with the same or a different filler material to form the secondelectrode. Alternatively, the support may be removed and then bothelectrodes formed simultaneously.

In another embodiment, the first electrode may or may not be perforated,and the support either is a second electrode or comprises an electricalcontact. Both electrodes may then be in contact with the entire array ofnanowires, enabling the measurement of the electrical property throughall of the nanowires.

A potential advantage of the method is the ability to form periodicperforations that are between the nanowires by an automatic process dueto the self-assembly of close-packed arrays of nanospheres. Noregistration or alignment process is required to site the perforations.Thus, the method may be scaled to large areas including entire waferswithout complications due to the size of the wafer.

Potential advantages of the structure are apparent in a gas sensor typeapplication where the geometry-enabled gas flow through the electrodeand nanowire array as well as the large number of vertical nanowiresconnected in parallel result in gas sensing with a fast response rateand high sensitivity. To achieve maximum gas flow throughout thestructure, a perforated top electrode layer can be very effective,whether the airflow is passive or actively pumped through the sensor.

Another feature of the nanosphere-enabled perforated electrode is thatthe properties of the holes in the top electrode, such as pitch anddiameter, can be easily controlled by simply varying the size of thenanospheres deposited atop the nanowires and changing the time for whichthey are etched down in oxygen plasma.

The electrodes disclosed herein can be used as a preconcentrator fordetection and partial separation of trace vapors. The Si NW arrays 1)serve as high surface area adsorptive substrates for trace vaporadsorption in a noncontact/standoff mode of operation, and 2) enablerapid and controlled Joule heating profiles that provide unique thermaldesorption spectra for component analysis by portable multichanneldetectors (i.e., mass spectrometer or ion mobility spectrometer). Bycoupling the trace vapor desorption output from the Si-NW to amultichannel detector and by deconvolving the data with chemometricmethodology, a correlation is established between the physicalproperties that determine the desorption of an analyte from an array ofnanowires and the analytical properties that define its successfulseparation and detection within a complex, unknowable background.

The use of Si NW arrays establishes a new paradigm for tunablesubstrates that efficiently preconcentrate (sensitivity enhancement) andpartially separate (selectivity enhancement) trace analytes in complexenvironments. The highly ordered silicon nanowire (Si-NW) arraysdescribed above provide a compact, powerful front end to diversemultichannel detectors that are either currently available or indevelopment. Differential sorption/desorption kinetics can be leveragedin a unique and powerful way to enhance selectivity while retaininginstrumental simplicity, by limiting the contribution of eddy diffusionas an analyte is delivered to a detector. The arrays can 1) serve ashigh surface area adsorptive substrates for trace explosives vaporadsorption in a noncontact/standoff mode of operation, and 2) enablerapid and controlled Joule heating profiles that provide unique thermaldesorption spectra for IED components. By coupling the trace explosivesvapor desorption output to an appropriate multichannel detector (e.g. amass spectrometer or ion mobility spectrometer) and taking advantage ofchemometric methodology, one can correlate the physical properties thatdetermine an the desorption of an analyte from a complex array ofnanowires with the analytical properties that define its successfulseparation and detection in a complex, unknowable background.

The method can result in numerous potential advantages over existingtechnologies. Using the Si NW array as both a preconcentrator andseparation medium reduces the overall complexity and size of traditionalanalytical instrumentation, and results in a significant drop in totalanalysis time. The form factor of the Si NW array facilitatesintegration with any number of multichannel or single channel detectors,for techniques including ion mobility spectrometry, mass spectrometry,optical methods, and combinations of sensor types. Joule heating of ahighly ordered array minimizes eddy diffusion during the desorptionprocess, resulting in improved sensitivity and selectivity for a givenanalyte. Moreover, the addition of a matrix of arrays with partiallyselective coatings to promote selective patterns of analyte adsorptionand desorption results in 3rd order instrumentation, which is onlyachievable commercially in multiple hyphenated instrumentation (i.e.,GC-GC-MS). The coating provides two general purposes—enhancedsensitivity and selectivity. The coating improves analyte retention onthe array based upon an analytes affinity for the coating. The nature ofthe coating also limits retention of analytes that do not match thosechemical properties, limiting adsorption and the potential forco-eluting interferents during desorption process. Generally,analyte/stationary phase interaction is governed by a number of forcesincluding, polarity, polarizability, hydrophobicity-hydrophilicity,hydrogen bonding, and acid-base characteristics. There are manymechanisms to generate coatings with these properties including, but notlimited to, liquid or vapor deposition polymerization, silanization, oradsorption. An analyte's interaction with a matrix of arrays withdifferent coatings, in terms of preconcentration capability andretention during desorption, would be unique to the molecule ofinterest.

The apparatus includes a nanowire structure, a current source, and adetector. The nanowire structure is as described above and includes theplurality of nanowires each having a first end in contact with a firstelectrode and a second end in contact with a second electrode. Thenanowires are perpendicular to the electrodes. The nanowires may be madeof any material that is warmed when current is passed through it. Onesuitable material is silicon.

The nanowires may include a chemically selective surface, such that notall compounds, or possibly only one compound, will adsorb onto thenanowires. Suitable surfaces include, but are not limited to, anadsorbing layer, a stationary phase such as is used in a chromatograph,and a surface functionalization, which can range from the sub-monolayerto materials that completely fill the interwire spaces. For example, aruthenia coating (RuO_(x)) may be used for detecting carbon monoxide.Functional groups may be added to silicon using compounds containing thefunctional group and alkoxy silane groups. Such functional groupsinclude, but are not limited to, hexafluoroisopropanyl to detect nitrogroups, pyrrole and thiophene to detect π-electron acceptors, andcarboxylic acid to detect organic bases.

An electrode may have perforations as described above. It may be acontinuous material and may be made from titanium and gold or othersuitable metals and metal combinations with appropriate chemicalreactivity and electrical conductivity. The current source iselectrically connected to the electrodes so that a current may be passedthrough the nanowires.

The apparatus may include more than one of the structures. The multiplestructures may include structures having different chemically selectivesurfaces.

The detector may be any detector capable of detecting the analyte vapor.Suitable detectors includes, but are not limited to, a massspectrograph, an ion mobility spectrograph, a fluorescent probe, acantilever, a chemiresistor, or a nanowire array as disclosed herein.The apparatus may also include a gas chromatograph for separating vaporsfrom the preconcentrator before they are detected.

The apparatus may be used by exposing the nanowire structure(s) to asample, such as ambient air, that is suspected of containing an analyte,such as an explosive vapor. The analyte adsorbs onto the nanowires. Aperiod of time is allowed to pass to increase the amount of analyte thatis adsorbed. Then the current source passes a current through thenanowires, which causes them to warm up by Joule heating. When thenanowire temperature is high enough, any adsorbed analyte desorbs into avapor that may be more concentrated than the original sample. Theanalyte may then optionally pass through a gas chromatograph, and thenpass to the detector.

The following examples are given to illustrate specific applications.These specific examples are not intended to limit the scope of thedisclosure in this application.

In one study, a combination of nanosphere lithography and metal-assistedchemical etching was used to synthesize well-ordered arrays of siliconnanowires (SiNWs) (Peng et al., Appl. Phys. Lett., 95 (2009) 243112).Silicon was chosen for its ease of fabrication and integration as wellas the wide availability of various functionalization and surfacemodification techniques for increased sensitivity and selectivity.Precise control over dopant type and concentration is available incommercially obtained wafers. The process started with a 100 mm diameterB-doped p-type Si(100) wafer of resistivity ˜10 Ω·cm that was cut into 1cm² pieces and successively cleaned in a 3:1 solution of H₂SO₄:H₂O₂(30%), 1:1:5 solution of H₂O₂(30%):NH₄OH:H₂O and deionized water. Theresulting hydrophilic substrate was then spin-coated (Cheung et al.,Nanotechnology 17 (2006) 1339-43) (FIG. 2A) with a close-packedmonolayer of 490 nm polystyrene nanospheres (Bangs Laboratories, 10%w/v). The nanospheres were subsequently reduced in diameter via anoxygen plasma etch (FIG. 2B). A perforated gold template for thecatalytic anisotropic etching of silicon was created by evaporating a 25nm thick layer of gold on top of the nanosphere array and subsequentlyremoving the nanospheres by soaking in CHCl₃ (FIG. 2C). The SiNWs werethen formed by immersing the device in a solution of 10% HF and 0.6%H₂O₂, where gold selectively and anisotropically etched into the siliconsubstrate, leaving behind a well-ordered array of vertically standingnanowires (FIG. 2D. A photoresist layer could be patterned over parts ofthe template to prevent the etching of silicon in certain locations,such as the contact pad region (FIG. 3). The silicon etch rate in theHF—H₂O₂ solution depends on multiple factors, including solutionconcentration, temperature, template dimensions, etc., but was shown tobe approximately 200 nm min⁻¹ in this case. The samples were typicallyetched for around 30 min to create up to ˜4×10⁸/cm² vertical SiNWs thatwere 4-6 μm in length and ˜200 nm in diameter, with ananowire-to-nanowire distance of 490 nm. The initial diameter of thepolystyrene nanospheres defined the SiNW array's period while thecombination of this initial diameter and subsequent etching of thenanospheres in oxygen plasma defined the resulting nanowire diameter.Next, a 500 nm thick layer of SiO₂ was evaporated over the entire deviceto electrically isolate the contact pad region from the bulk of thesubstrate. The oxide layer was then selectively etched away to revealthe SiNW array while removing any residual oxides on the nanowiresurfaces. This step also decreased the contact resistance andestablished ohmic contact between the nanowire tips and the electrodelayer deposited later. The entire SiNW array was then covered with athick photoresist that was subsequently etched back in oxygen plasma toreveal just the SiNW tips (FIG. 2E).

After exposing the SiNW tips, a second layer of nanospheres identical tothe ones used earlier in making the etch template was deposited. Sincethe period of this second nanosphere layer was equal to the period ofthe SiNWs, the new nanospheres were physically constrained to perfectlyoccupy the voids in the array and form a close-packed array on top ofthe exposed SiNW tips. After slightly etching down the second nanospherearray in an oxygen plasma, evaporating a metal electrode layerconsisting of 20 nm thick titanium and 100 nm thick gold, and finallyremoving the photoresist and nanospheres with acetone, a large SiNWarray (5 mm×5 mm) with a PTE layer was formed as seen in FIG. 2G. Somepolystyrene nanospheres are still visible in and are the result of localvariations in photoresist and gold film thickness. The size anddistribution of pores could be controlled by varying the nanosphereprocessing conditions, and the contact resistance between the nanowiresand the top electrode could be reduced even further by performing alow-temperature anneal. The completed devices were mounted on pin gridarray (PGA) packages using a conductive epoxy to make the bottomelectrical connections. Top electrical connections were made bywirebonding to the contact pads (FIG. 3).

To evaluate the chem/biosensing capabilities of the PTE SiNW arraysensors, the completed devices were exposed to varying levels of NO₂ orNH₃ in a custom-built testing chamber (Field et al., Anal. Chem. 83(2011) 4724-4728). A dual manifold (an analyte line and a clean airline) was constructed out of coated stainless steel (SilcoNert CoatedStainless Steel Tubing, Restek) to minimize wall adsorption. Compressedgas cylinders of ammonia and nitrogen dioxide were connected to theanalyte line of the manifold. A zero air generator (Environics) andhumidity control unit (Miller-Nelson) were used to create humidified air(˜40% relative humidity) for both the analyte and clean air lines of themanifold. The known concentrations of the analyte were achieved bydiluting calibrated gas standards (100 ppm ammonia and 50 ppm nitrogendioxide, Airgas) with the carrier air via a T-connector and mass flowcontroller. A three-way valve and actuator were used to switch betweenthe clean and analyte lines of the manifold. The entire manifold wasplaced in a temperature controlled oven. A stainless steel samplechamber with a cone geometry was built for testing PGA-mounted sensors.A sample pump was used to flow air through the chamber at 100 mL/min.

Electrical connections within the sample chamber were made with azero-insertion force (ZIF) socket and a simple printed circuit board foreasy loading and unloading of sensors. A multiplexer (Keithley, 2001)and source-meter (Keithley, 2602) were connected to the circuit board ofthe sample chamber. The multiplexer allowed for selection of specificpins and functions of the PGA and sensor, respectively. Resistance wasmonitored by sourcing 100 μA of current and recording the voltage at asample rate of 10 Hz. The sensor electronics were monitored andcontrolled by a Lab VIEW program. The resistance recorded duringexposure to clean air was averaged to obtain the initial resistance, R₀.The sensor response (ΔR/R₀) was calculated as the difference inresistance (R−R₀, ΔR) normalized by the initial resistance (R₀) forcomparison and further evaluation. All data modeling and plotting wereperformed using the OriginPro 8.1 software package.

Without further treatment or modification of silicon, surface adsorptionof electron-withdrawing (donating) species like NO₂ (NH₃) decreases(increases) the overall resistance of the p-type Si devices. Asignificant distinction of the this vapor delivery system is that it canmix the analytes of interest with a calibrated amount of humidified airas opposed to dry N₂ to simulate a real-world testing environment.Sensor testing in humidified air is a crucial step towards real-worldimplementation because SiNWs are highly sensitive to water vapors.

The prototype sensors were tested for response to varying concentrationsof NO₂ or NH₃ at a controlled temperature of 40° C. and relativehumidity of ˜30%. The change in resistance was determined by holding aconstant current of 10 μA while recording voltage with a voltmeter.Sensor response was plotted as the change in resistance divided by thebaseline resistance (ΔR/R₀), without any filtering or smoothing of theraw, real-time data. FIG. 4A shows the response of the prototype sensorsto 1 ppm and 500 ppb of NO₂ and NH₃ in humidified air, respectively. Asexpected, total device resistance increased when exposed to NH₃ anddecreased upon exposure to NO₂. The response reached saturation within afew minutes likely due to the PTE while the massively parallel nanowireconfiguration resulted in a very low noise profile. Humidified airadversely affects NO₂/NH₃ detection capabilities in metal oxide (Starkeet al., Sensors and Actuators B, 2002, 239-45) and carbon nanotube(Zhang et al., Nanotechnology 20 (2009) 255501) sensors. However, waterappears to improve the sensor response at very low analyteconcentrations. For detection at lower concentrations, the humiditylevel in the testing chamber was reduced to <10% RH. Sensor responsefollowing 30 min of exposure to 250, 50, and 10 ppb of NO₂ is shown inFIG. 4B. For the lowest concentration level of 10 ppb, the sensorexhibited an 18% drop in resistance; 10 ppb sensitivity to NO₂ is amongthe lowest ever reported for a SiNW-based sensor and is far belowvarious international and national requirement standards for annual NO₂exposure (Belanger et al., Am. J. Resp. Crit. Care Med. 173 (2006)297-303).

The effect of the PTE on sensing performance was investigated byomitting the second nanosphere deposition step in the fabricationprocess to produce sensors with solid, nonporous electrodes. The deviceswith and without holes in the electrode layer were identical in allother aspects. The sensing response of both types of devices to 500 ppbof NH₃ is shown in FIGS. 5A-B. Both sensors reached similar saturationlevels over time, but the PTE sensors, represented by the top line,reached this level in approximately 6 min. The non-porous variety, onthe other hand, required almost 1 h to reach saturation. The response toNO₂ was also faster for the PTE sensors, albeit not as pronounced aswith NH₃. This difference is explained by the parallel electricalconfiguration of the nanowires and the different resistance changesinduced by the interacting molecules. NH₃ induces a resistance increase,so most of the nanowires must change for a large overall response by thearray. In contrast, NO₂ decreases the individual nanowire resistance, soonly a few nanowires can cause a large change in resistance for theentire array. For all detection schemes, but in particular for thoseresulting in increased nanowire resistance, the holes in the topelectrode layer significantly improve detection response by allowing theanalytes to flow directly through the electrode layer to quicklyinteract with all the nanowires in the array. The relative sensitivityto analyte electronegativity could be reversed by fabricating thenanowires from n-doped Si.

In another experiment, a total of six sensors from a single batch weretested. The sensors were initially exposed to clean air for 2 min,followed by exposure to either ammonia or nitrogen dioxide for 8 min. Anadsorption-based sensor should follow a Langmuir adsorption model and bemass-transport limited; thus, the resistance should changeasymptotically (Washburn et al., Anal. Chem. 81 (2009) 9499-9506;Washburn et al., Anal. Chem. 82 (2011) 69-72; Eddowes et al., Biosensors3 (1987) 1-15; Bunimovich et al., J. Am. Chem. Soc. 128 (2006)16323-16331). The 8 min exposure time was used to determine the fullrise time of the sensor response for both ammonia and nitrogen dioxide.

The responses to ammonia or nitrogen dioxide at 40° C. at differentconcentrations are shown in FIG. 6. The data presented are from onerepresentative sensor; results from additional sensors were generallyconsistent. The slight elevation in temperature eliminatedtemperature-induced fluctuations in sensor response. The concentrationsof ammonia and nitrogen dioxide ranged from 250 ppb to 10 ppm. From FIG.6, as noted and expected, the resistance increased for ammonia anddecreased for nitrogen dioxide. The response saturated (leveled off) atapproximately 10 min run time (8 min exposure time) for both analytes,regardless of concentration. However, the sensor needed at least 1 h ofclean air exposure to partially desorb the analyte from the nanowiresurfaces and return to a stable, flat baseline at 40° C. (data notshown). Because of irreversible adsorption of analytes on the nanowires,the baseline never fully recovered to its original, pre-exposureresistance but reached a new equilibrium resistance and over time thesensor lost sensitivity. The incomplete desorption of analyte from thenanowire surface during exposure limited the number of exposures andprevented replicate measurements for each concentration of ammonia ornitrogen dioxide. The recovery and lifetime can probably be improvedwith a higher operating temperature since adsorption/desorption istemperature dependent but is a trade-off with sensitivity and requiresadditional optimization. Thermal desorption of the analyte could easilybe accomplished to regenerate the sensor by passing an electricalcurrent through the wires, resulting in Joule heating and a rise intheir temperature.

FIG. 6 shows the resistance change for exposure to 10 ppm ammonia,including a maximum during the initial exposure. This initial maximum isonly observed at relatively high ammonia concentrations and is mostpronounced at 10 ppm. No initial maximum is observed for nitrogendioxide at any concentration, which suggests that it is analytespecific. For example, ammonia and humidified air could react to formammonium hydroxide. Alternatively, ammonia may dissociate to NH₂ and Hon the silicon surface, as has been observed at room temperature inultrahigh vacuum (Bozso et al., Phys. Rev. Lett. 57 (1986) 1185; Dillon,J. Vac. Sci. Technol., A 9 (1991) 2222). Dissociation would change thechemistry or restructure the silicon nanowire surface and could make theremaining surface less reactive. While the source of the initial maximumhas not been definitively identified, its presence does not hinderadditional analysis of the silicon nanowire-based sensor's overallresponse and performance.

FIG. 6 shows the rapid response as a sharp increase in resistance afterexposure to ammonia following a 2 min exposure to clean air. Theseconds-to-minutes saturation response of the silicon nanowire-basedsensor is remarkable because the sensor is at near-room-temperature andhumidified air is used as the carrier, as opposed to dry air or an inertgas. A direct comparison between sensors with porous and solid topelectrodes confirmed that the porosity enables the rapid response.Modeling and simulations of the conical sample chamber (data not shown)indicate a uniform vapor front is delivered through the PTE over theentire sensor surface, thereby reducing the diffusion time for theanalyte molecules to traverse the wire array.

The signal-to-noise ratio of the silicon nanowire-based sensor ismarkedly improved over comparable nanotube and nanowire-based sensors(Peng et al., Appl. Phys. Lett. 95 (2009) 243112; Lee et al., J. Phys.Chem. B 110 (2006) 11055-11061; Snow et al., Chem. Soc. Rev. 35 (2006)790-798; Snow et al., Nano Lett. 5 (2005) 2414-2417; Robinson et al.,Nano Lett. 8 (2008) 3137-3140). The signal-to-noise ratio wasapproximately 1000:1 for both of the analytes tested in humidified,near-room temperature air (FIG. 6). This result was obtained at a samplerate of 10 Hz and required no post-acquisition smoothing, filtering, orbackground subtraction. The excellent analyte response and minimalbackground humidity response are attributable to the PTE and the factthat every nanowire in the array is in electrical contact with both thetop and bottom electrodes. Other vertically aligned nanowire-basedsensors have relatively small electrodes in contact with only a fractionof the unordered nanowires, so only a small number of the nanowires actas sensing elements (Peng et al., Appl. Phys. Lett. 95 (2009) 243112).The PTE in the present sensor ensures that every nanowire is a sensingelement in a massively parallel array that minimizes noise sourcessensitive to the number of charge carriers, e.g., 1/f noise. Shot noiseat the interface between the nanowires and the PTE was further minimizedby removing the native oxide layer from the tips of the nanowires.

The initial slope method has been effectively used for adsorption-basedsensors as a means of obtaining quantitative information, but notably inthe liquid phase and for non-nanowire-based sensors (Washburn et al.,Anal. Chem. 81 (2009) 9499-9506; Washburn et al., Anal. Chem. 82 (2010)69-72; Eddowes, Biosensors 3 (1987) 1-15). An initial slope methodallows for shorter sampling times without the need to achieve saturationand can yield a more linear calibration curve over a larger dynamicrange. The sensor response at each concentration of ammonia and nitrogendioxide in FIG. 6 was fitted to a single exponential function(y=Ae^(−t/r)+y₀). The slope at t=0, which is the time when the valve isswitched to the analyte line, is simply, A/r. FIG. 7, panels A and Bshow calibration curves for ammonia and nitrogen dioxide, respectively,where the initial slope (A/r) is plotted versus concentration on a ln-lnscale.

A fixed-time point method using |ΔR/R₀|_(saturation), where|ΔR/R₀|_(saturation) is the normalized response at 10 min run time, wasalso used to establish calibration curves for comparison (FIG. 7, panelsC and D). The R² is 0.996 and 0.912 for the initial slope method and0.711 and 0.807 for the fixed-time point method. The relative predictionerror (RPE), which is the average of the error associated with eachcalculated concentration in the calibration curve, for ammonia andnitrogen dioxide is 5.1% and 24.9% for the initial slope method comparedto 49.0% and 40.3% for the fixed time point method, respectively. Undermass-transport limited conditions, the initial slope exhibits a powerlaw dependence that correlates better with concentration than afixed-time point at saturation. The ammonia calibration curve isreasonable considering the curve fitting does not explicitly model theinitial maximum observed at higher concentrations, but the nitrogendioxide calibration curve can still be improved, perhaps with a betterfitting model than a single exponential.

The initial slope method provides a better correlation to concentrationthan the fixed-time point method because it eliminates sensorsaturation. This not only reduces sampling times and makes the sensormore applicable to real-world environments but also improves sensorrecovery and lifetime by limiting the amount of material needed forquantitation and the amount that must be desorbed to regenerate thesensor.

A preconcentrator was also made. A self-assembled layer of polystyrenebeads was spin coated onto the silicon substrate, and reactive ionetching was used to reduce the bead size, defining the final nanowirediameter and spacing. A 25 nm gold metal layer was deposited, and thebeads were then removed via solvent. The polystyrene beads act as ananomask, leaving the gold film pierced by an array of holes equal insize to the beads. Next, metal-assisted chemical etching (MACE) was usedto create the highly ordered nanowire array. Briefly, the sample wasimmersed in a solution of hydrofluoric acid (HF) and hydrogen peroxide,whereupon a redox reaction occurred at the interface between the goldand the silicon. The silicon oxidized and the oxide was removed by theHF. As a result, the gold film effectively “sinks” into the Si wafer;wherever there is a hole in the film, a Si nanowire is formed.Fabrication was completed by the deposition of a 2^(nd) porous goldelectrode on top of the array, which allows vapor to penetrate into thearray in a top down fashion. The porous top electrode allows one to passa current and heat the SiNWs while also allowing analytes to passquickly into the array during sampling and out of the array duringdesorption.

The temperature-dependent shift in the Si Raman single phonon line wasused to estimate the temperature of the SiNWs as a function of appliedcurrent. The relationship between the one-phonon Raman peak location andtemperature rise in Si is approximately linear from room temperature to600K (Balkanski et al, “Anarmonic Effects in Light Scattering Due toOptical Phonons in Silicon,” Phys. Rev. B. Vol. 28, pp. 1928-1934,1983). At room temperature, the one-phonon Raman peak occurs at ˜520cm⁻¹. For every 1 K increase in temperature, the Raman peak shiftdecreases by about 0.02 cm⁻¹. It should also be noted the Raman peakbroadens with temperature.

FIG. 8 shows an SEM side view of the SiNW array. FIG. 9 shows an opticalimage of the array. The Si substrate is located at the bottom of theimage, and the SiNWs are located between the white dashed lines. TheRaman spectrum as a function of current was collected from a few SiNWslocated the laser focus spot (bright spot in the middle of the image).Current was run through the SiNWs by applying a voltage between theporous top electrode and the Si substrate. The Raman spectrum forapplied currents ranging from 0 mA up to 120 mA is shown in FIG. 10.Note that as the current increased, the Raman peak red-shifted andbroadened, indicating a rise in the SiNW temperature. The spectralintensity also decreased with increasing current. It is known that theRaman peak intensity decreases with temperature, but part of thedecrease observed may also be due to thermal drift.

Each Raman FIG. 10 was fitted with a Gaussian line shape. The centerwavenumber of the Gaussian fit is plotted in FIG. 11 as a function ofapplied current. A linear fit of the Raman peak center wavenumber vs.current yields a slope of −0.037 cm⁻¹/mA. To convert the Raman centerposition to SiNW temperature, the following equation was used:temperature=(R _(RT) −R(I))/0.02 cm⁻¹/° C.+22° C.where R_(RT) is the Raman center position at room temperature (i.e.,applied current=0 mA) and R(I) is the Raman center position at anapplied current of I. A room temperature of 22° C. was assumed. A linearfit of the temperature vs. current yields a slope of 1.78° C./mA.

Trace 2,4-DNT vapors were delivered to SiNW arrays using a custom madevapor handling system. Briefly, a calibrated permeation tube of 2,4-DNTwas placed in a permeation oven and operated per manufacturersspecifications to produce a nominal mass flux of 2,4-DNT per unit time.The volume flow rate of air through the permeation oven was 1 L/min. Thenominal vapor concentration was attenuated by controlled flow ofpurified air, allowing for total flow rates of 3.5 to 21 L/min or vaporconcentration range from 28.4 to 4.7 ppb_(v).

The desorption of 2,4-DNT from the array was detected using an Agilent5976 mass selective detector (MSD). Briefly, the sample chambercontained a stainless steel base and a ZIF socket to which the SiNWarray, mounted on a pin grid array chip, was connected. A Plexiglas topsealed directly to the pin grid array chip. Multiple access portsallowed the introduction and exit of 2,4-DNT vapor, and access to theMSD by a heated capillary transfer line.

Arrays were evaluated by delivering a known concentration of 2,4-DNTvapor to the array at a particular flow rate and duration, resulting inthe delivery of a nominal mass of 2,4-DNT to the sample chamber. Aftersample loading, desorption programs were initiated using a customLabVIEW program controlling a Keithley 2602A SourceMeter.

A representative desorption “chromatogram” is shown in FIG. 12. In thisinstance, over 200 ng of 2,4-DNT was delivered to the array. Desorptioncurrent was applied, starting with 10 mAmps for 8 seconds, and increasedto 200 mAmps at 30 second intervals. The peak area of 2,4-DNT plateauedat approximately 160 mAmps. The concentration of 2,4-DNT vapor,estimated from the chromatogram, was in excess of 1000 ppb.

This work demonstrates the successful preconcentration and desorption of2,4-DNT from SiNW arrays. The array's large surface array enablesanalyte adsorption on the surface without a stationary phase. Rapiddesorption occurs when the nanowires are Joule heated by passing acurrent through them. This work suggests the feasibility of thisapproach for inclusion as an integrated preconcentration stage for newlaboratory and portable analytical devices.

In a similar fashion, a mixture of nitrobenzene (NB) and 2,4-DNT wasdelivered to the Si NW array. Nominal vapor concentrations were 30ppb_(v) and 4.5 ppb_(v), respectively. Sample flow rates and time weresuch that the nominal mass load to the array was 30 ng of NB and 7 ng of2,4-DNT. At t=1 minute, desorption current was applied for approximately10 seconds, resulting in the desorption of NB and 2,4-DNT from thearray. Results are depicted in FIG. 13.

In a similar fashion, a mixture of nitrobenzene (NB), 2,6-DNT, and2,4-DNT was delivered to the Si NW array. Nominal vapor concentrationswere 30 ppb_(v), 9.9 ppb_(v) and 4.5 ppb_(v), respectively. Sample flowrates and time were such that the nominal mass load to the array was 30ng of NB, 15 ng for 2,6-DNT, and 7 ng of 2,4-DNT. At t=1 minute,desorption current was applied for approximately 10 seconds resulting inthe desorption of the three components from the array. Results aredepicted in FIG. 14.

In a similar fashion, a mixture of 2,6-DNT and TNT was delivered to theSi NW array. Nominal vapor concentrations were 10 ppb_(v), and 8ppb_(v), respectively. Sample flow rates and time were such that thenominal mass load to the array was 60 ng of each analyte. At t=0.5minute, desorption current was applied for approximately 10 seconds,resulting in the desorption of the two components from the array.Results are depicted in FIG. 15.

Numerous potential applications and device configurations exist,including, but not limited to, 1) the use of a matrix of SiNW arrayswith different sorbent materials resulting in the generation ofdesorption “chromatograms” that vary as a function of analyte affinityto the sorbent material, 2) the use of Si NW to provide meter doses ofvapor on demand, by first overloading the Si NW array with sample andbriefly heating the array to only deliver a small portion of theadsorbed analyte. This application is suitable for vapor detectorcalibration.

Obviously, many modifications and variations are possible in light ofthe above teachings. It is therefore to be understood that the claimedsubject matter may be practiced otherwise than as specificallydescribed. Any reference to claim elements in the singular, e.g., usingthe articles “a,” “an,” “the,” or “said” is not construed as limitingthe element to the singular.

What is claimed is:
 1. A method comprising: providing two or morestructures, each structure comprising: a first electrode; a plurality ofnanowires comprising a chemically selective surface perpendicular to thefirst electrode, each nanowire having a first end in contact with thefirst electrode; and a second electrode in contact with a second end ofeach nanowire; wherein different structures have chemically selectivesurfaces that are selective for different analytes; exposing thestructures to a sample suspected of containing an analyte that canadsorb onto the nanowires of at least one of the structures; and passingan electrical current through the nanowires to heat the nanowires to atemperature at which the analyte will desorb from the nanowires.
 2. Themethod of claim 1, wherein the second electrode comprises perforations.3. The method of claim 2, wherein the nanowires and the perforations areperiodically arranged.
 4. The method of claim 1, wherein the nanowirescomprise silicon.
 5. The method of claim 1, wherein the chemicallyselective surface is an adsorbing layer, a stationary phase, or asurface functionalization.
 6. The method of claim 1, wherein the secondelectrode is a continuous material.
 7. The method of claim 1, whereinthe second electrode comprises titanium and gold.
 8. The method of claim1, further comprising: detecting any desorbed analyte.
 9. The method ofclaim 8, wherein the detection is by mass spectroscopy, ion mobilityspectrometry, change in fluorescence intensity of a fluorescent probe,change in resonance of a cantilever, change in frequency of acantilever, change in resistance of a chemiresistor, or detection by ananowire array.
 10. The method of claim 8, further comprising: passingthe desorbed analyte through a gas chromatograph before detecting thedesorbed analyte.
 11. The method of claim 8, wherein the detection is bymass spectroscopy, ion mobility spectrometry, change in fluorescenceintensity of a fluorescent probe, change in resonance of a cantilever,change in frequency of a cantilever, or detection by a nanowire array.12. The method of claim 1, wherein there are no electrodes between thesides of the nanowires.
 13. An apparatus comprising: two or morestructures, each structure comprising: a first electrode; a plurality ofnanowires comprising a chemically selective surface perpendicular to thefirst electrode, each nanowire having a first end in contact with thefirst electrode; and a second electrode in contact with a second end ofeach nanowire; wherein different structures have chemically selectivesurfaces that are selective for different analytes; a current sourceelectrically connected to each of the first electrodes and each of thesecond electrodes; and a detector configured to detect an analyte thatmay be desorbed from the nanowires.
 14. The apparatus of claim 13,wherein the chemically selective surface is an adsorbing layer, astationary phase, or a surface functionalization.
 15. The apparatus ofclaim 13, wherein the detector is a mass spectrograph, an ion mobilityspectrograph, a fluorescence probe, a microcantilevers, a chemiresistor,or a nanowire array.
 16. The apparatus of claim 13, further comprising:a gas chromatograph configured to pass the analyte from the nanowires tothe detector.
 17. The apparatus of claim 13, wherein there are noelectrodes between the sides of the nanowires.
 18. The apparatus ofclaim 13, wherein the detector is a mass spectrograph, an ion mobilityspectrograph, a fluorescence probe, a microcantilevers, or a nanowirearray.